Glossary of Terms

• Also see Acronyms

Airport planning and the Environmental Impact Statement (EIS) process require the use of many technical terms.  Some of the most important terms are identified and defined in this list.

Terms in bold italics are also defined separately in this glossary.

Above Ground Level (AGL) - A measurement of altitude above a specific land mass.

Air Route Traffic Control Center (ARTCC or Center) - A Federal Aviation Administration facility established to provide air traffic control service to aircraft operating on Instrument Flight Rules flight plans within controlled airspace during the en route portion of flight.

Air Traffic Control (ATC) - A service operated to promote the safe, orderly, and expeditious flow of air traffic.

Airport Traffic Control Tower (ATCT) - An air traffic control facility that has been established on an airport to provide for safe, orderly, and expeditious flow of traffic on and in the vicinity of the airport.

Aircraft Rescue and Fire-Fighting (ARFF) - The department at an airport dedicated to protecting life and property, controlling fire hazards, and performing general duties related to airport operations and aircraft safety.

Airman's Information Manual (AIM) - A publication containing basic flight information and air traffic control procedures, designed primarily as a pilot's information and instructional manual for use in the National Airspace System.

Airport Elevation - The highest point on an airport's usable runways, expressed in feet above mean sea level.

Airport Improvement Program (AIP) - A federal funding program for airport improvements.  AIP is periodically reauthorized by Congress with funding appropriated from the Aviation Trust Fund.  Proceeds to the Trust Fund are derived from excise taxes on airline tickets, aviation fuel, etc.

Airport Layout Plan (ALP) - A scaled drawing of existing and proposed land and facilities necessary for the operation and development of the airport.  The ALP shows boundaries and proposed additions to all areas owned or controlled by the airport operator for airport purposes, the location and nature of existing and proposed replacement airport facilities and structures, and the location on the airport of existing and proposed non-aviation areas and improvements thereon.

Airport Operations - Takeoffs (departures) and landings (arrivals) from an airport.

Airport Surveillance Radar (ASR) - A radar system which allows air traffic controllers to identify an arriving or departing aircraft's distance and direction from an airport.

Airway - A corridor of controlled airspace whose centerline is established by radio navigational aids.  Low altitude airways (between 3,000 and 18,000 feet Mean Sea Level) are identified by number with the letter V as a prefix.  High altitude airways (above 18,000 feet MSL) are known as Jet airways and are identified by number with the letter J as a prefix.

Ambient Noise - The total sum of noise from all sources in a given place and time. 

Approach Light Systems (ALS) - A series of lights that assists the pilot when aligning aircraft with the extended runway centerline on final approach.

Area Navigation (RNAV) - A method of navigational procedures designed to transition aircraft between an airport environment and the en route system of airspace.  RNAV procedures offer the advantages of routings that save time and fuel, reduce dependence on radar vectoring, altitude, and speed assignments, which allows for reduction in required radio transmissions with Air Traffic Control, and more efficient use of airspace.

Attenuation - Acoustical phenomenon whereby sound energy is reduced between the noise source and the receiver.  This energy loss can be attributed to atmospheric conditions, terrain, vegetation, other natural features, and man-made features (e.g., sound insulation).

A-Weighted Sound (dBA) - A system for measuring sound energy that is designed to represent the response of the human ear to sound.  Energy at frequencies more readily detected by the human ear is more heavily weighted in the measurement, while frequencies less well detected are assigned lower weights.  A-weighted sound measurements are commonly used in studies where the human response to sound is the object of the analysis.

Azimuth - An arc of the horizon measured between a fixed point (such as true north) and the vertical circle passing through the center of an object.

Base leg - A flight path at right angles to the approach of a runway end.  It usually extends from the downwind leg to the intersection of the extended runway centerline.  See Traffic Pattern.

Baseline Condition - The existing condition or conditions prior to future development, which serve as a foundation for analysis.

Commuter Aircraft - Commuter aircraft range from small turboprop aircraft with 19 or fewer seats to Regional Jets with up to 70 seats.  Although Regional Jets that seat up to 90 passengers are sometimes referred to as "commuter jets" because they tend to serve the same types of markets as smaller jets, they cannot be operated by Federal Aviation Regulations Part 135 commuter carriers.

Contour - See Noise Contour.

Controlled Airspace - Airspace of defined dimensions within which air traffic control service is provided in accordance with the airspace classification.  Controlled airspace is designated as Class A, Class B, Class C, Class D, or Class E.  Aircraft operators are subject to certain pilot qualifications, operating rules, and equipment requirements as specified in Federal Aviation Regulations Part 91, depending upon the class of airspace in which they are operating.

Council on Environmental Quality (CEQ) - Federal entity that serves as the principal environmental policy adviser to the President and oversees federal agency implementation of the environmental impact assessment process.

Crosswind leg - A flight path at right angles to the approach runway end off of its upwind end.

Day-Night Average Sound Level (DNL) - A noise measure used to describe the average sound level over a 24-hour period, typically an average day over the course of a year.  In computing DNL, an extra weight of 10 decibels is assigned to noise occurring between the hours of 10:00 p.m. and 7:00 a.m. to account for increased annoyance when ambient noise levels are lower and people are trying to sleep.  DNL may be determined for individual locations or expressed in noise contours.

dBA - See A-weighted Sound Level

Decibel (dB) - Sound is energy and is measured by its pressure.  Because of the enormous range of sound pressures to which the human ear is sensitive, the raw sound pressure measurement is converted to the decibel scale for purposes of description and analysis.  The decibel scale is logarithmic.  A 10-decibel increase in sound is perceived as a doubling of sound (or twice as loud) by the human ear.

Displaced Threshold - A threshold that is located at a point on the runway other than the designated beginning of the runway.  The portion of pavement behind a displaced threshold may be available for takeoffs in both directions and landings from the opposite direction.

Distance Measuring Equipment (DME) - A flight instrument that measures the line-of-sight distance of an aircraft from a navigational radio station in nautical miles.

Double-Clear Zone - An area on the ground, up to 1,250 feet from each side of the runway centerline and extending 5,000 feet beyond each end of the primary runway surface.  It is also known as the Approach Transitional Area for runways serving or anticipated to serve turbojet aircraft or having an existing or planned runway with an instrument approach.

Easement - The legal right of one party to use part of the rights of a piece of real estate belonging to another party.  This may include, but is not limited to, the right of passage over, on or below the property; certain air rights above the property, including view rights; and the rights to any specified form of development or activity.

Enplanements - The number of passengers boarding an aircraft at an airport.

En Route System - That part of the National Airspace System where aircraft are operating between origin and destination airports.

En Route Control - The control of Instrument Flight Rules traffic en route between two or more adjacent approach control facilities.

Environmental Assessment (EA) - A concise document that assesses the environmental impacts of a proposed federal action.  It discusses the need for, and environmental impacts of, the proposed replacement airport and alternatives.  An environmental assessment should provide sufficient evidence and analysis for a federal determination whether to prepare an Environmental Impact Statement or issue a Finding of No Significant Impact.  Public participation and consultation with other federal, state, and local agencies is a cornerstone of the EA process.

Environmental Impact Statement (EIS) - An EIS is a document that provides a discussion of the significant environmental impacts which would occur as a result of a proposed replacement airport, and informs decision-makers and the public of the reasonable alternatives which would avoid or minimize adverse impacts.  Public participation and consultation with other federal, state, and local agencies is a cornerstone of the EIS process.

Equivalent Sound Level (Leq) - The A-weighted energy average sound level experienced over a given period of time.  The metric is expressed as 10 times the log of the total noise energy divided by the number of seconds during the period under consideration.

Federal Aviation Administration (FAA) - The FAA is the federal agency responsible for insuring the safe and efficient use of the nation's airspace, for fostering civil aeronautics and air commerce, and for supporting the requirements of national defense.  The activities required to carry out these responsibilities include: safety regulations, airspace management and the establishment, operation and maintenance of a system of air traffic control and navigation facilities; research and development in support of the fostering of a national system of airports, promulgation of standards and specifications for civil airports, and administration of federal grants-in-aid for developing public airports; various joint and cooperative activities with the Department of Defense, and technical assistance (under State Department auspices) to other countries.

Federal Aviation Regulations (FAR) - The body of federal regulations relating to aviation; published as Title 14 of the Code of Federal Regulations.

Final Approach - A flight path that follows the extended runway centerline.  It usually extends from the base leg to the runway.

Finding of No Significant Impact (FONSI) - If, following the preparation of an environmental assessment, the responsible federal agency determines a proposed replacement airport would not result in any significant environmental impact, a FONSI is issued by the federal agency.  A FONSI is a document briefly explaining the reasons why an action would not have a significant effect on the human environment and for which an Environmental Impact Statement, therefore, is not necessary.

Fix - A geographical point in airspace.

Fixed-Base Operator (FBO) - A business located on the airport that provides services such as hangar space, fuel, flight training, repair, and maintenance to airport users.

Fleet Mix - The mix or differing types of aircraft operating in a particular environment.

Flight Level (FL) - The exact altitude, to the nearest 100 foot increment, at which an aircraft is flying during the enroute portion of the flight. Flight Level information is used by air traffic control for aircraft separation purposes.

Flight Tracks - Established routes for arrival and departure by aircraft to and from the runways at an airport.

Flight Management System (FMS) - Equipment onboard an aircraft that takes advantage of various radio navigation and/or Global Positioning System routes to guide the aircraft along its intended departure, enroute, and arrival courses.

Fly-By Waypoint - A waypoint is a predetermined geographical position defined in terms of latitude/longitude (earth-based). A fly-by waypoint is used to delineate when an aircraft should begin a turn to the next course prior to reaching the waypoint separating the two route segments.  This can best be described as turn anticipation.

Fly-Over Waypoint - A waypoint is a predetermined geographical position defined in terms of latitude/longitude (earth-based). A fly-over waypoint is used when the aircraft must fly over the precise point prior to starting a turn.

Glide Slope (GS) - Provides vertical guidance for aircraft during approach and landing.  The glide slope consists of the following:

Electronic components emitting signals which provide vertical guidance by reference to airborne instruments during instrument approaches such as Instrument Landing System, or visual ground aids, such as Visual Approach Slope Indicator, which provide vertical guidance for visual flight rules approach or for the visual portion of an instrument approach and landing.

Geographic Information Systems (GIS) - An information system that is designed for storing, integrating, manipulating, analyzing, and displaying data referenced by spatial or geographic coordinates.

Global Positioning System (GPS) - A system of 24 satellites used as reference points to enable navigators equipped with GPS receivers to determine their latitude, longitude, and altitude.

Grid Analysis - A type of aircraft noise analysis that evaluates the noise levels at individual points rather than through generation of noise contours.

Ground Effect - Noise attenuation attributed to absorption or reflection of noise by man-made or natural features on the ground surface.

Hub - An airport that services airlines that have hubbing operations.

Hubbing - A method of airline scheduling that times the arrival and departure of several aircraft in a close period of time in order to allow the transfer of passengers between different flights of the same airline in order to reach their ultimate destination.  Several airlines may conduct hubbing operations at an airport.

Instrument Approach - A series of predetermined maneuvers for the orderly transfer of an aircraft under instrument flight rules from the beginning of the initial approach to a landing, or to a point from which a landing may be made visually.

Instrument Flight Rules (IFR) - That portion of the Federal Aviation Regulations (14 CFR 91) specifying the procedures to be used by aircraft during flight in Instrument Meteorological Conditions.  These procedures may also be used under visual conditions and provide for positive control by Air Traffic Control. (See also Visual Flight Rules).

Instrument Landing System (ILS) - An electronic system installed at some airports which helps to guide pilots to runways for landing during periods of limited visibility or adverse weather.

Instrument Meteorological Conditions (IMC) - Weather conditions expressed in terms of visibility, distance from clouds, and cloud ceilings during which all aircraft are required to operate using Instrument Flight Rules (IFR).

Integrated Noise Model (INM) - A computer model developed, updated and maintained by the Federal Aviation Administration to predict the noise exposure generated by aircraft operations.

Itinerant Operation - An aircraft flight that ends at an airport different from where it began.

Knots - Airspeed measured as the distance in nautical miles (6,076.1 feet) covered in one hour. (Approximately equal to 1.15 miles per hour.)

Land Use Compatibility - The ability of land uses surrounding the airport to coexist with airport-related activities with minimum conflict.

Landing and Takeoff (LTO) Cycle - The time that an aircraft is in operation at or near an airport.  An LTO cycle begins when an aircraft starts its final approach (arrival) and ends after the aircraft has made its climb-out (departure).

Ldn - See DNL.  Ldn is used in place of DNL in mathematical equations only.

Leq - See Equivalent Sound Level.

Local Operation - An aircraft flight that begins and ends at the same airport.

Localizer - The component of an Instrument Landing System that provides lateral course guidance to the runway.

Loudness - The subjective assessment of the intensity of sound.

Maximum Noise Level (Lmax) - The maximum sound pressure for a given event adjusted toward the frequency range of human hearing.

Mean Sea Level (MSL) - The average height of the surface of the sea for all stages of the tide; used as a reference for elevations; also called sea level datum.

Military Operations Area (MOA) - Airspace established to separate or segregate certain non-hazardous military activities from Instrument Flight Rules traffic and to identify for Visual Flight Rules traffic where these activities are conducted.  

Missed Approach - A prescribed procedure to be followed by aircraft that cannot complete an attempted landing at an airport.

National Airspace Redesign (NAR) - A multi-year initiative to review, redesign, and restructure the nation's airspace to meet the rapidly changing and increasing operational demands on the National Airspace System (NAS).

National Airspace System (NAS) - The common network of U.S. airspace, air navigation facilities, equipment, services, airports, or landing areas; aeronautical charts, information, and services; rules, regulations, and procedures; technical information, manpower, and materials, all of which are used in aerial navigation.

National Environmental Policy Act of 1969 (NEPA) - The original legislation establishing the environmental review process for proposed federal actions.

Nautical Mile - A measure of distance equal to one minute of arc on the earth's surface (6,076.1 feet or 1,852 meters).

Navigational Aids (NAVAIDs) - Any facility used by an aircraft for navigation.

Navigational Fix - A geographical position determined by reference to one or more radio navigational aids.

Noise Abatement - A measure or action that minimizes the amount of impact of noise on the environs of an airport.  Noise abatement measures include aircraft operating procedures and use or disuse of certain runways or flight tracks.

Noise Contour - A map representing average annual noise levels summarized by lines connecting points of equal noise exposure.

Non-directional Beacon (NDB) - A beacon transmitting non-directional signals whereby the pilot of an aircraft equipped with direction finding equipment can determine the bearing to and from the station.  When the radio beacon is installed in conjunction with the Instrument Landing System marker, it is normally called a compass locator.

Non-precision Approach - A standard instrument approach procedure providing runway alignment but no glide slope or descent information.

Operation - A takeoff or landing by an aircraft.

Outer Fix - An air traffic control term for a point in the airspace from which aircraft are normally cleared to the approach fix or final approach course.

Outer Marker (OM) - An Instrument Landing System navigation facility in the terminal area navigation system located four to seven miles from the runway edge on the extended centerline indicating to the pilot that he/she is passing over the facility and can begin final approach.

Positive Control - The separation of all air traffic within designated airspace as directed by air traffic controllers.

Precision Approach Path Indicator (PAPI) - Provides visual approach slope guidance to aircraft during an approach.  It is similar to a Visual Approach Slope Indicator but provides a sharper transition between the colored indicator lights.

Precision Approach Procedure - A standard instrument approach procedure in which an electronic glideslope/glidepath is provided (e.g., Instrument Landing System and Precision Approach Radar).

Precision Approach Radar (PAR) - Navigational equipment located on the ground adjacent to the runway, consisting of one antenna, which scans the vertical plane, and a second antenna, which scans the horizontal plane.  The PAR provides the controller with a picture of the descending aircraft in azimuth, distance, and elevation, permitting an accurate determination of the aircraft's alignment relative to the runway centerline and the glide slope.

Prevailing Wind - The direction of the wind that blows most frequently across a particular region.

Profile - The position of the aircraft during an approach or departure in terms of altitude above the runway and distance from the runway end.

Propagation - Sound propagation is the spreading or radiating of sound energy from the noise source.  It usually involves a reduction in sound energy with increased distance from the source.  Atmospheric conditions, terrain, natural objects, and manmade objects affect sound propagation.

Public Use Airport - An airport open to public use without prior permission, and without restrictions within the physical capabilities of the facility.  It may or may not be publicly owned.

Radar Vectors - See Vectors.

Record of Decision (ROD) - The official notice of the Federal Aviation Administration's findings after review of a Draft Environmental Assessment or Environmental Impact Statement.

Regional Jet (RJ) - A jet aircraft typically, but not exclusively, operated by a commuter air carrier ranging in size from approximately 35 to 80 seats.  Regional Jets were introduced in the early 1990s as 50 seat aircraft but their popularity with airlines and passengers resulted in aircraft manufacturers extending their product lines to include jets currently ranging from 35 to 90 seats.  There are new generation Regional Jets currently under production that will have 100 seats.

Reliever Airport - An airport which, when certain criteria are met, relieves the aeronautical demand on a busier air carrier airport.

Retrofitted Aircraft - An aircraft originally certified as Stage 2 that has been modified to meet Stage 3 requirements.  This includes both modification of engines or the replacement of engines to meet the Stage 3 standard.

RNAV - See Area Navigation

Run-up - A routine procedure for testing aircraft systems by running one or more engines at a high power setting.  Engine run-ups are normally conducted by airline maintenance personnel checking an engine or other on board systems following maintenance.

Runway End Identifier Lights (REIL) - Two synchronized flashing lights, one on each side of the runway threshold, which identify the approach end of the runway.

Runway Protection Zone (RPZ) - An area, trapezoidal in shape and centered about the extended runway centerline, designated to enhance the safety of aircraft operations.  It begins 200 feet (60 M) beyond the end of the area usable for takeoff or landing.  The RPZ dimensions are functions of the aircraft, type of operation, and visibility minimums. (Formerly known as the clear zone).

Runway Safety Area (RSA) - A defined surface surrounding the runway prepared or suitable for reducing the risk or damage to airplanes in the event of an undershoot, overshoot, or excursion from the runway.

Runway Threshold - The beginning of that portion of the runway usable for landing.

Scoping - Scoping is an early and open process for determining the scope or range of issues to be addressed in the Environmental Impact Statement and identifying the significant issues related to a proposed action.  Issues important to the public and local, state, and federal agencies are solicited through direct mailing, public notices, or meetings. 

Single event - One noise event.  For many kinds of analysis, the sound from single events is expressed using the Sound Exposure Level metric.

Slant-range distance - The distance along a straight line between an aircraft and a point on the ground.

Sound - Sound is the result of vibration in the air.  The vibration produces alternating bands of relatively dense and sparse particles of air, spreading outward from the source in the same way as ripples do on water after a stone is thrown into it.  The result of the movement is fluctuation in the normal atmospheric pressure or sound waves.

Sound Exposure Level (SEL) - A standardized measure of a single (sound) event, expressed in A-weighted decibels, that takes into account all sound above a specified threshold set at least 10 decibels below the maximum level.  All sound energy in the event is integrated over one second.

Special Use Airspace - Airspace of defined dimensions identified by an area on the earth's surface wherein activities must be confined because of their nature and/or wherein limitations may be imposed upon aircraft operations, which are not part of those activities.

Stage 2 Aircraft - Aircraft that meet the noise levels prescribed by Federal Aviation Regulations Part 36, which are less stringent than those established for the quieter Stage 3 designation.  The Airport Noise and Capacity Act required the phase-out of all Stage 2 aircraft over 75,000 pounds by December 31, 1999, with the potential for case-by-case exceptions through the year 2003.

Stage 3 Aircraft - Aircraft that meet the most stringent noise levels set in Federal Aviation Regulations Part 36.

Standard Instrument Approach Procedures - see U.S. Terminal Procedures Publication.

Standard Instrument Departure Procedure (SID) - A planned Instrument Flight Rules air traffic control departure procedure published for pilot use in graphic and textual form.  SIDs provide transition from the terminal to the en route air traffic control structure.

Standard Terminal Arrival Route (STAR) - A planned Instrument Flight Rules air traffic control arrival procedure published for pilot use in graphic and textual form.  STARs provide transition from the en route air traffic control structure to an outer fix or an instrument approach fix in the terminal area.

Statute Mile - A measure of distance equal to 5,280 feet.

Tactical Air Navigation (TACAN) - A navigational system used by the military.  TACAN provides both azimuth and distance information to a receiver on board an aircraft.

Terminal Radar Approach Control (TRACON) - A Federal Aviation Administration Air Traffic Control Facility which uses radar and two-way communication to provide separation of air traffic within a specified geographic area in the vicinity of one or more airports.

Terminal Radar Service Area (TRSA) - Airspace surrounding certain airports where Air Traffic Control provides radar vectoring, sequencing, and separation on a full-time basis for all Instrument Flight Rules and participating Visual Flight Rules aircraft.

Time Above (TA) - The amount of time that sound exceeds a given decibel level during a 24-hour period (e.g., time in minutes that the sound level is above 75 decibels).

Turbojet - An aircraft powered by a jet turbine engine.  The term is customarily used in air traffic control for all aircraft, without propellers, that are powered by variants of jet engines, including turbofans.

Turboprop - An aircraft powered by a jet turbine engine that drives a propeller.  Aircraft of this type are typically used by airlines on short routes between two relatively close locations.

Traffic Pattern - The traffic flow for aircraft landing and departure at an airport.  Typical components of the traffic pattern include: upwind leg, crosswind leg, downwind leg, base leg, and final approach.

UNICOM - A non-government communication facility, which may provide airport information at certain airports.  Aeronautical charts and publications show the locations and frequencies of UNICOMs.

Upwind Leg - A flight path parallel to the approach runway in the direction of approach.

Uncontrolled Airspace - Airspace of defined dimensions within which air traffic control service is not provided or is not deemed necessary.  Uncontrolled airspace is designated as Class G. Aircraft operators are subject to certain pilot qualifications, operating rules, and equipment requirements as specified in Federal Aviation Regulations Part 91, depending upon the class of airspace in which they are operating.

Vector - Compass heading instructions issued by Air Traffic Control in providing navigational guidance by radar.

Very High Frequency Omnidirectional Range (VOR) Station - A ground-based radio navigation aid transmitting signals in all directions.  A VOR provides azimuth guidance to pilots by reception of electronic signals.

Very High Frequency Omnidirectional Range Station with Tactical Air Navigation (VORTAC) - A navigational aid providing VOR azimuth and Tactical Air Navigation distance measuring equipment  at one site.

Visual Approach - An approach conducted on an Instrument Flight Rules flight plan, which authorizes the pilot to proceed visually and clear of clouds to the airport.

Visual Approach Slope Indicator (VASI) - A visual aid for final approach to the runway threshold, consisting of two wing bars of lights on either side of the runway.  Each bar produces a split beam of light - the upper segment is white, the lower is red.

Visual Flight Rules (VFR) - Rules and procedures specified in Federal Aviation Regulations Part 91 for aircraft operations under visual conditions.  Aircraft operations under VFR are not generally under positive control by Air Traffic Control.  The term VFR is also used in the U.S. to indicate weather conditions that are equal to or greater than minimum VFR requirements.  In addition, it is used by pilots and controllers to indicate a type of flight plan.

Visual Meteorological Conditions (VMC) - Weather conditions expressed in terms of visibility, distance from cloud, and cloud ceiling equal to or greater than those specified in Federal Aviation Regulations Part 91.155 for aircraft operations under Visual Flight Rules.

Waypoint - A predetermined geographical position that is defined in terms of latitude/longitude coordinates. See also Fly-By Waypoint and Fly-Over Waypoint.

Yearly Day-Night Average Sound Level - see DNL.